1. Field of the Invention
The invention pertains generally to closed loop fuel management systems having an oxygen sensor positioned in the exhaust gas of an internal combustion engine for sensing the constituent makeup of the exhaust gas and is more particularly directed to optimization techniques for fuel economy while utilizing such fuel management systems.
2. Prior Art
There are many and diverse examples of air/fuel ratio controllers for internal combustion engines in the art. Generally, controllers of this type adjust or regulate the amount of fuel mixed with the ingested air of an internal combustion engine to provide the most optimal mixture of the two for burning in the cylinders. Numerous operating parameters of the engine may be sensed and applied to various schedules to calculate the ratio desired but generally speed, manifold absolute pressure, and the temperature of the engine and ingested air are the most useful.
The air/fuel mixture normally desired or scheduled for is stoichiometric or .lambda.=1.0 because of the attendant advantages of relatively good fuel economy and emission control. Close control of the air/fuel ratio around the stoichiometric point is required of such controllers when using three-way catalytic converters as they will operate efficiently only in a very narrow window of air/fuel ratios. It is also known that extended lean operation of an engine will damage some converters and cause a decrease in conversion efficiency. The air/fuel ratio controller with its open loop schedule is therefore a facile method of maintaining an air/fuel ratio close to stoichiometric for an electronic fuel injector system, electronic carburetor apparatus, or other air/fuel regulating devices.
It is evident, however, that an open loop schedule will not be correct at all times for all engines manufactured because of tolerances, wear, maintenance, changing ambient conditions and other variable criteria. To make these open loop schedulers provide even a more precise control of air/fuel ratio self adaptive or closed loop control has been applied to the open loop schedulers. An advantageous closed loop system that controls the air/fuel ratio at stoichiometric enjoying considerable success is one which includes an oxygen sensor position in the exhaust gas of the engine controlled.
The oxygen sensor provides two voltage levels where one is relatively high indicative of a relative absence of oxygen in the exhaust gas or a rich air/fuel ratio and the other is relatively low indicative of a substantial presence of oxygen in the exhaust gas or a lean air/fuel ratio. The switching between the two levels occurs with a relatively rapid slope at the stoichiometric point as the air/fuel ratio passes therethrough.
By providing an integral control law based upon this switching point, a limit cycle oscillation is produced wherein the air/fuel ratio goes below and above stoichiometric in a narrow band whose average is stoichiometric. An example of a closed loop fuel management control system of this type utilizing an O.sub.2 sensor is disclosed in a U.S. Pat. No. 3,815,561 issued to Seitz on June 11, 1974 which is commonly assigned with the present application. The disclosure of Seitz is herein expressly incorporated by reference.
Since the slope of the sensor signal when switching at stoichiometric is not infinite, some variation away from stoichiometric, either rich or lean, can be obtained in the average air/fuel ratio by comparing the sensor voltage with a threshold indicative of the air/fuel ratio desired. The variation is, however, unduly limited by the slope of the sensor waveform at the switching point and the better the sensor (steeper slope) the less the variation obtainable. Moreover, the air/fuel ratios desired and set by the threshold will be unreliable as the sensor ages and the characteristic curve of the sensor changes. A constant adjusting of the system will be required to maintain a predetermined or desired air/fuel ratio. A threshold system for operating a closed loop O.sub.2 integral controller is disclosed in a U.S. Pat. No. 3,874,171 issued to Schmidt et al on Apr. 1, 1975.
Another system advantageously describes the use of asymmetrical integration for operating a closed loop O.sub.2 system at rich or lean air/fuel ratios with a stoichiometric sensor. This system is more fully disclosed in a U.S. Pat. No. 4,099,491 entitled "System For Controlling Any Air/Fuel Ratio With Stoichiometric Sensor and Asymmetrical Integration" in the name of J. N. Reddy and commonly assigned with the present application. The disclosure of Reddy is herein expressly incorporated by reference.
These systems then could provide a means for controlling air/fuel ratio for specific conditions. For example, during many operational times an internal combustion engine may be operated more economically at a leaner air/fuel ratio than stoichiometric. At constant cruise conditions when there are no abnormal loads or acceleration demands, the engine will run smoothly at air/fuel ratios of approximately 18:1 or higher. It would require less fuel for the operation of the engine if in addition to operating at an average air/fuel ratio that is stoichiometric, a closed loop system could switch to different average air/fuel ratios in response to the sensing of certain conditions but still maintain the desirable precise control afforded by an O.sub.2 sensor.